fix: conditionally clean memory based on environment variable

mineru/backend/pipeline/model_json_to_middle_json.py

 # Copyright (c) Opendatalab. All rights reserved.
+import os
 import time
 
 from loguru import logger
@@ -235,6 +236,7 @@ def result_to_middle_json(model_list, images_list, pdf_doc, image_writer, lang=N
 
     """清理内存"""
     pdf_doc.close()
+    if os.getenv('MINERU_DONOT_CLEAN_MEM') is None:
         clean_memory(get_device())
 
     return middle_json